The present invention relates to surface roughening methods and more particularly to a method for electrochemical roughening of thin film electrodes for increasing active surface area, decreasing electrode impedance, increasing charge injection capacity, increasing sensitivity of biosensors and improving adhesion to substrates.
Thin film microfabrication techniques have enabled miniaturization and reproducibility of electrodes used in neurorecording and neuromodulation chronic biomedical devices as well as in biosensors. However, the reduction in an electrode's geometric surface area can compromise important electroactive characteristics of electrodes used for a wide range of biomedical applications, such as for example, 1) diminishes the amount of charge that can be safely delivered by the electrode during neuro-stimulation; 2) shows undesirable increases in impedance for recording electrodes; and 3) decreases electrochemical signals measured with biosensors.
In order to develop devices with greater spatial resolution while maintaining the same electroactive functionality, microelectrodes must be able to have enhanced performance compared to the performance of macroelectrodes. Generally, this may be accomplished by 1) depositing thin film coating of different material with enhanced electrochemical activity over the surface of the electrode; or 2) increasing the effective surface area of the electrode while keeping the geometric surface area the same. In the case of depositing a different material electroactive thin film coating over the electrode to improve performance of an electrode, however, this often results in poor adhesion of the deposited film to the electrode surface. And poor film adhesion leads to absence of mechanical robustness of implantable device which may result in immediate delamination or decreased lifetime of the electrode upon implantation.
In many cases poor adhesion can be greatly improved by roughening of a substrate prior to film deposition. Several methods are known for increasing the effective surface area for a given geometric surface area, including for example: (1) roughening a planar electrode through some form of etching, e.g. by a physical etching of the surface using some form of plasma with inert gases or chemical etching with an acid or base, (2) depositing a rough, high-surface-area electrode material on top of a flat or roughened surface of a planar electrode by electrochemical deposition, e.g. electroplating, electrophoretic deposition, through binding/casting of nano- or microparticles, or through chemical vapor deposition, sputtering, or evaporation of nano- or microparticles. In addition, methods for roughening of thick metal and foil electrodes are also known. Platinum, gold, and palladium thick metal and foil electrodes can be roughened electrochemically by application of short bipolar pulses with oxide forming during anodic pulse and oxide etching during cathodic pulse. Electrochemical etching methods are highly attractive because of high increase in surface area, good process control, low cost and low toxicity. Unfortunately, these methods are not applicable to roughening thin films, since films often fail mechanical stress introduced during electrochemical etching process and loose mechanical integrity. Thus, existing roughening methods known to work for thick metals and metal foils are not directly applicable for thin film electrodes and often results in film delamination from the substrate.
High surface area electrodes are desirable for applications that rely on electrical charge delivery, low impedance, and improved adhesion between substrate and electroplated films. There is therefore a need to increase the effective or active surface area of a thin film electrode to increase the electrode charge injection capacity, without increasing the geometric area of the microfabricated thin film electrode. Moreover, there is also a need for surface roughening method that may be used for roughening thin film electrodes without causing delamination.